As well known in the arts, polycarbonate resins have been extensively applied to the production of various molded products. For example, substrates for optical recording media such as optical discs or magneto-optical discs have been produced mainly by injection-molding these polycarbonate resins. In order to injection-mold such precision products, it has been required to reduce the amount of volatile oligomers which are derived from the polycarbonate resins and which are adhered to stampers or dies. However, the volatilization of the polycarbonate-derived oligomers has a close relationship to the content thereof in the polycarbonate resins. There are hitherto known no polycarbonate resins which can be considerably reduced in content of the oligomers, except for extracted products obtained by using a solvent.
Conventionally, in order to improve properties of polycarbonate resins, intense efforts have been directed to studies for developing such a method for producing polycarbonate resins having not only a small content of oligomers but also a narrow molecular weight distribution. For example, in Japanese Patent Application Laid-Open (KOKAI) No. 55-52321(1980), there has been described a method comprising reacting bisphenol A with phosgene at a low temperature to produce oligomers having a narrow molecular weight distribution, and then polymerizing the thus obtained oligomers in the presence of an end sealing agent, thereby producing a polycarbonate resin having a high molecular weight and a narrow molecular weight distribution. In Japanese Patent Application Laid-Open (KOKAI) No. 1-278528(1989), there has been described a method of producing a polycarbonate resin having a narrow molecular weight distribution by conducting a two-stage polymerization in which bisphenol A and phosgene are first reacted with each other in the presence of an end sealing agent to produce oligomers, and then bisphenol A is added to the thus obtained oligomers. In Japanese Patent Application Laid-Open (KOKAI) No. 3-109420(1991), there has been described a method comprising reacting bisphenol A with phosgene in a first pipe reactor, adding an end sealing agent to the reaction solution, and then introducing the reaction solution through a second pipe reactor into a vessel-type reactor where the reaction solution is further reacted to produce an oligomer of polycarbonate. In this KOKAI, it has been further described that the thus obtained polycarbonate oligomer is further mixed with bisphenol A and subjected to a two-stage polymerization, thereby producing a polycarbonate resin having a narrow molecular weight distribution.
In Japanese Patent Application Laid-Open (KOKAI) Nos. 6-336522(1994) and 7-165899(1995), there has been described a method of producing a polycarbonate resin having not only a small content of oligomers but also a narrow molecular weight distribution, comprising reacting bisphenol A with phosgene in the absence of an end sealing agent to obtain a prepolymer, and then adding an end sealing agent to the obtained prepolymer to conduct the interfacial polymerization thereof. In these KOKAIs, it has also been described that polycarbonate resins having a molecular weight distribution (Mw/Mn) of about 2.0 were obtained. However, the descriptions of these KOKAIs are deemed to be inconsistent with that of the above Japanese Patent Application Laid-Open (KOKAI) No. 3-109420(1991), i.e., such a description that when the addition of an end sealing agent is delayed, the molecular weight distribution of the obtained polycarbonate resin becomes broader. In fact, when the molecular weight distributions (Mw/Mn) of the polycarbonate resins obtained in these KOKAIs are calculated from glass transition temperatures thereof according to the relationship between glass transition temperature and molecular weight as described in "Eur. Polym. J.", vol. 18, pp. 563 to 567(1982), it has been recognized that the Mw/Mn values are far larger than 2 and almost close to 3. For this reason, it is suggested that the actual molecular weight distributions of the polycarbonate resins described in these KOKAIs are considerably broad when precisely measured.
These conventional methods for the production of polycarbonate resins all have aimed at reducing the content of low-molecular weight oligomers, thereby preventing volatile components derived therefrom from being generated upon heat-molding. However, any of these methods has failed to completely remove the low-molecular weight oligomers from the obtained polymer. Further, there have been proposed methods of producing relatively suitable materials by reprecipitation or extraction using a poor solvent (Japanese Patent Application Laid-Open (KOKAI) Nos. 63-278929(1988), 64-6020(1989) and 4-306227(1992)). However, these methods not only have failed to completely remove oligomers from the obtained materials, but also require additional steps for separating the solvent used from the materials, inevitably resulting in increased costs.
Also, as another method for modifying the melt-flowability of polycarbonate resins, there has been studied such a method of sealing end groups of a polycarbonate molecule with compounds having a long-chain alkyl group such as long-chain alkyl phenol or long-chain alcohol (refer to British Patent No. 965,457, U.S. Pat. No. 3,240,756, and Japanese Patent Application Laid-Open (KOKAI) Nos. 51-34992(1976) and 60-203632(1985)). Further, many condensation catalysts have been reported (refer to U.S. Pat. Nos. 3,160,606, 3,173,891, 3,184,431, 3,240,756 and 3,275,601). Any of these prior arts has aimed at improvement in reaction rate, and requires the use of an end sealing agent in combination.
Therefore, an object of the present invention is to provide a low-volatile polycarbonate resin having an extremely narrow molecular weight distribution and containing substantially no low-molecular weight oligomers.